Tracking artificial satellites by radio interferometers is known in the art. One of the earliest systems for this purpose was the Minitrack System developed by the Naval Research Laboratory discussed in the article "Tracking Satellites by Radio" by John T. Mengel and Paul Herget published in the January 1958 issue of Scientific American, Volume 198, Number 1, page 23.
Radio interferometers known in the art obtain the angular position of a radio source by measuring the difference in radio path lengths from a source to each of a group of antennas. For instance, FIG. 1 shows two antennas, 12 and 14, erected on a baseline which is precisely measured in length, location and direction. From the known measured length of this baseline, its radio path length is known to be n wavelengths. The signal from the radio source 11 arrives at antenna 12 at the same time as it arrives at point 16. Thus, the distance 15 is the difference in radio path length between radio paths to 12 and 14 equal to x wavelengths. When the source 11 is at great distance, the angle 18 is a right angle, and the cosine of angle 19 is x.lambda./n.lambda. or x/n. If a measurement of the relative phase of the signals arriving at 12 and 14 is made, the value of x, and hence angle 19 would be determined. However, if the path length difference exceeds one wavelength, ambiguous readings will result. Such ambiguities are resolved by using multiple pairs of antennas for each baseline direction, arranged so that the shortest baseline does not produce a radio path length difference exceeding one wavelength anywhere within the beam pattern of the antennas used. The outputs of each of these antenna pairs are then processed to fine tune the calculation.
The addition of another baseline, normal to the first, provides a second angle to the radio source. All of the angle measurements are made as direction cosines. Accordingly, two such measurements allow the third direction cosine to be calculated, to determine the angular position of the radio source.
In practice however, the radio or sky waves provided by a radio source such as an artificial satellite are characterized by multiple propagation paths from radio source to the receiving or sending location. For the case of a transmitting source on the earth's surface, stratification in the ionosphere causes the typical sky wave to arrive at one or more elevation angles by single, double or multiple hop modes which may include one or more ground reflections. The ionosphere also causes polarization rotation and doppler variation as a function of time. These time-varying phenomena are sometimes very pronounced and very rapid and can cause erroneous readings at the receiving location.
Obviously, it is desirable and necessary to obtain radio wave phase data which is more accurate to calculate the exact position of the radio source. Accordingly, it is a primary object of the present invention to provide a system and method for obtaining the aforesaid more accurate data.